can't get MIDI tracks into the mixdown

I’ve done a fair few purely audio mixdowns in the past, and one mixdown that included a preset Groove Agent 1 track to which i’d added a few beats of my own using MIDI keyboard through GA1. The mixdown was no problem and all tracks came down into the mix.

But now I have a project that consists of a few audio tracks plus a few MIDI tracks that have been recorded through VST instruments (Halion and a Groove agent1 track). When I do the mixdown I get a file that contains only the audio tracks and none of the MIDI tracks. All the MIDI tracks were recorded through VST instruments using a basic MIDI keyboard (M-Audio Key Studio) then modified using the editor.

So the question is how do I get my MIDI tracks into the mixdown? I haven’t found anything in the manual to help me with this. there isn’t even an index entry for mixdown of tracks of any kind that i can see though as i say i’ve managed many previously with no problem. what am i missing? (all answers gratefully accepted)

Ok I set audio tracks to Bus 1 instead of stereo output and it mixed down beautifully.

Wish I knew why.